a) Field of the Invention
The present invention relates to a vari-focal lens system for cameras, especially vide cameras.
b) Description of the Prior Art
The lens systems currently used with domestic video cameras are mostly designed for specifications of zooming ratios of 6 to 10 and aperture ratios of F/1.2 to F/2.0. This is because these specifications are most convenient for designing the lens systems and suited for satisfying customers' demands.
Each of these lens systems is generally composed of four lens units as exemplified by those disclosed by Japanese Patents Kokai Publication No. Sho 58-102208 and No. Sho 58-153913.
Such a vari-focal lens system is generally composed, in the order from the object side, of a first lens unit which has a positive refractive power, is kept fixed for variation of focal length and has a focusing function, a second lens unit which has a negative refractive power and is movable for variation of focal length and has a vari-focal function, a third lens unit which has a function to correct displacement of image surface caused by variation of focal length, an aperture stop, and a fourth lens unit which has a positive refractive power, is always kept fixed and has an imaging function.
This type of vari-focal lens system comprising the four lens units is suited for enhancing vari-focal ratio and enlarging aperture size. However, due to the fact that the first lens unit has the positive refractive power, the vari-focal lens system is unsuited for widening field angle and can have field angles thereof are limited to approximately 50.degree. at maximum at the wide position thereof.
The vari-focal lens systems which are currently placed on the market and composed of four lens units have small field angles which do not permit satisfactory photography indoors. Accordingly, users of these lens systems desire vari-focal lens systems having wider field angles.
On the other hand, vari-focal lens systems each composed of two lens units are available as lens systems having wide field angles. Each of these vari-focal lens systems generally consists, in the order from the object side, of a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, and is adapted so as to vary focal length thereof by moving these lens units while varying an airspace reserved therebetween.
Further, this vari-focal lens system in which the negative lens unit is arranged before the positive lens unit is suited for widening the field angle thereof, but unsuited for enhancing vari-focal ratio and enlarging aperture size thereof, and generally has a vari-focal ratio on the order of 2.
Furthermore, in the vari-focal lens system composed of the two lens units, an aperture stop is arranged in the second lens unit and generally moved together with the second lens unit for varying focal length. The aperture stop movable with the lens unit is undesirable since it complicates structure of a lens barrel and increase manufacturing cost of the lens system.
As vari-focal lens systems for video cameras which are designed for wide field angles, there are known the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 63-292106 and Japanese Patent Kokoku Publication No. Hei 1-191820.
The former vari-focal lens system is composed of three lens units. Since this vari-focal lens system adopts an aperture stop which is moved together with the second lens unit, it is undesirable in that the lens barrel has a complicated structure and F number thereof is varied by variation of focal length.
The latter vari-focal lens system is composed of three lens units which are moved for varying focal length, comprises an aperture stop which is fixed between the second lens unit and the third lens unit, and has a vari-focal ratio of 2 to 3 which is not sufficiently satisfactory.
Moreover, as another example of the vari-focal lens system each consisting of three lens units, there is known the lens system proposed by Japanese Patent Kokoku Publication No. Sho 64-40913. This lens system has a vari-focal ratio a little lower than 3 which is neither sufficiently satisfactory. In addition, this vari-focal lens system has a field angle of 45.degree. at maximum at the wide position thereof, and cannot be said as a wide field angle lens system.
As is understood from the foregoing description, the conventional vari-focal lens systems have a common defect that field angles are narrow at the wide position when vari-focal ratios are high, or vari-focal ratios are low when field angles are wide at the wide position.